With large-sized refrigeration equipment, if the pressure inside the refrigeration equipment abnormally increases or the temperature of refrigerant increases, the refrigeration equipment itself may be damaged, and high pressure gas may spew out to damage the surroundings. As a mechanism for preventing damage and breakage of refrigerators, large-sized refrigeration equipment is required to have a safety device for controlling the pressure of refrigerant gas in the refrigeration equipment (Rule 7-1-8 of Refrigeration Safety Regulations of the Ministry of Economy, Trade, and Industry of Japan). Such equipment is typically provided with a safety device such as a fusible plug. Safety devices for refrigeration equipment are designed by each manufacturer in accordance with the refrigerant being used. In the past, freon and CFC (chlorofluorocarbon) refrigerants were the most widely used refrigerants in refrigeration equipment. However, CFC refrigerants can undergo photolysis in the stratosphere due to ultraviolet light in sunlight to form active chlorine, causing the problem that the ozone layer is destroyed by the active chlorine. Therefore, strict restrictions on the use of these refrigerants have been developed on a global level. Accordingly, they are now being replaced by HCFC (hydrochlorofluorocarbon) refrigerants which are replacements for freon. HFC (hydrofluorocarbon) refrigerants which have a small ozone depletion potential have also been developed. Thus, different types of refrigerants are being developed and used in refrigeration equipment.
Fusible plugs used in refrigeration equipment need to be designed based on the refrigerant which is used. When the pressure of refrigerant used in refrigeration equipment increases, the temperature of the refrigerant increases in accordance with Boyle's law. Therefore, the operating temperature of a fusible plug being used is determined in accordance with the condensing pressure of the refrigerant being used. For example, in the case of refrigerating equipment for air conditioning which employs R22 (HCFC 22) (which is the HCFC refrigerant which currently is most in demand) as a refrigerant for refrigeration equipment, the condensing pressure is 1.94 MPa and the critical temperature of R22 is 96.2° C. Thus, the operating temperature of a fusible plug is designed to be approximately 95-100° C.
The critical temperature varies with the refrigerant being used. When the refrigerant is changed, it is necessary to redesign the operating temperature of a fusible plug to be used in refrigeration equipment. When using R407C, which is a HFC refrigerant used as a replacement refrigerant having a small ozone depletion potential, the condensing pressure is 2.11 MPa and the critical temperature is 85.6° C., so a fusible plug having a design temperature of approximately 90-95° C. can be used. In the case of a fusible plug for use with R410a (which is a HCFC refrigerant having a good compression efficiency) as a refrigerant for refrigerating equipment, the condensing pressure is 3.06 MPa and critical temperature is 71.5° C. Since the critical temperature of the refrigerant rises, it is necessary to design refrigeration equipment so that the design temperature of the fusible plug is approximately 70-75° C.
Fusible plugs for refrigeration equipment use low melting point solder alloys, and solder alloys containing the harmful substances Pb and Cd have been employed. When R22 is used as a refrigerant, since the design temperature of a fusible plug is 96° C., Sn-52Bi-32Pb (eutectic point of 96° C.) has been used. When R410a is used as a refrigerant, the design temperature of a fusible plug becomes 70-75° C., and a solder alloy such as Sn-50Bi-10Cd-26.7Pb (solidus temperature of 69° C., peak temperature of 76° C., and liquidus temperature of 81° C.) has been used.
Fusible plugs are recovered together with refrigeration equipment. When refrigeration equipment is discarded, it is of course necessary to treat the equipment in accordance with laws and regulations. Particularly in recent years, active efforts are being made to protect the global environment, and there is a tendency to exclude harmful components from parts used in equipment such as refrigeration equipment. In particular, Cd and Pb components have a harmful effect on the human body, and they are becoming the subject of regulations.
Fusible plugs which do not contain harmful components such as Cd and Pb include ones using an alloy of two or more substances selected from Sn, Bi, In, Zn, and Ga (JP 2002-115940 A1), low melting point alloys for fusible plugs characterized by being a tin-indium-bismuth based alloy in which when the composition is Sn: X weight %, In: Y weight %, and Bi: Z weight %, then X+Y+Z=100 and 4≦X≦10, and 56≦Y≦63 (JP 2001-214985 A1), ones in which fine metal particles are added to a fusible alloy comprising bismuth, indium, and tin (JP 2003-130240), and the like.    Patent Document 1: JP 2002-115940 A1    Patent Document 2: JP 2001-214985 A1    Patent Document 3: JP 2003-130240 A1